1. Field of the Invention
The present invention relates to secondary (rechargeable) alkali-ion batteries. More specifically, the invention relates to materials for use as electrodes for an alkali-ion battery. The invention provides transition-metal compounds having the ordered olivine or the rhombohedral NASICON structure and containing the polyanion (PO4)3xe2x88x92 as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.
2. Description of the Related Art
Present-day lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. On discharge, the metallic anode supplies Li+ ions to the Li+-ion electrolyte and electrons to the external circuit. The cathode is typically an electronically conducting host into which Li+ ions are inserted reversibly from the electrolyte as a guest species and charge-compensated by electrons from the external circuit. The chemical reactions at the anode and cathode of a lithium secondary battery Must be reversible. On charge, removal of electrons from the cathode by an external field releases Li+ ions back to the electrolyte to restore the parent host structure, and the addition of electrons to the anode by the external field attracts charge-compensating Li+ ions back into the anode to restore it to its original composition.
Present-day rechargeable lithium-ion batteries use a coke material into which lithium is inserted reversibly as the anode and a layered or framework transition-metal oxide is used as the cathode host material (Nishi et al., U.S. Pat. No. 4,959,281). Layered oxides using Co and/or Ni are expensive and may degrade due to the incorporation of unwanted species from the electrolyte. Oxides such as Li1+x[Mn2]O4, which has the [M2]O4 spinel framework, provide strong bonding in three dimensions and an interconnected interstitial space for lithium insertion. However, the small size of the O2xe2x88x92 ion restricts the free volume available to the Lixe2x88x92 ions, which limits the power capability of the electrodes. Although substitution of a larger S2xe2x88x92 ion for the O2xe2x88x92 ion increases the free volume available to the Li+ ions, it also reduces the output voltage of an elementary cell.
A host material that will provide a larger free volume for Li+-ion motion in the interstitial space would allow realization of a higher lithium-ion conductivity "sgr"Li, and hence higher power densities. An oxide is needed for output voltage, and hence higher energy density. An inexpensive, non-polluting transition-metal atom would make the battery environmentally benign.
The present invention meets these goals more adequately than previously known secondary battery cathode materials by providing oxides containing larger tetrahedral is oxide polyanions forming 3D framework host structures with octahedral-site transition-metal oxidant cations, such as iron, that are environmentally benign.
The present invention provides electrode material for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte. The cell may additionally include an electrode separator. As used herein, xe2x80x9celectrochemical cellxe2x80x9d refers not only to the building block, or internal portion, of a battery but is also meant to refer to a battery in general. Although either the cathode or the anode may comprise the material of the invention, the material will preferably be useful in the cathode.
Generally, in one aspect, the invention provides an ordered olivine compound having the general formula LiMPO4, where M is at least one first row transition-metal cation. The alkali ion Li+ may be inserted/extracted reversibly from/to the electrolyte of the battery to/from the interstitial space of the host MPO4 framework of the ordered-olivine structure as the transition-metal M cation (or combination of cations) is reduced/oxidized by charge-compensating electrons supplied/removed by the external circuit of the battery in, for a cathode material, a discharge/charge cycle. In particular, M will preferably be Mn, Fe, Co, Ti, Ni or a combination thereof Examples of combinations of the transition-metals for use as the substituent M include, but are not limited to, Fe1xe2x88x92xMnx, and Fe1xe2x88x92xTix, where 0 less than x less than 1.
Preferred formulas for the ordered olivine electrode compounds of the invention include, but are not limited to LiFePO4, LiMnPO4, LiCoPO4, LiNiPO4, and mixed transition-metal compounds such as Li1xe2x88x922xFe1xe2x88x92xTixPO4 or LiFe1xe2x88x92xMnxPO4, where 0 less than x less than 1. However, it will be understood by one of skill in the art that other compounds having the general formula LiMPO4 and an ordered olivine structure are included within the scope of the invention.
The electrode materials of the general formula LiMPO4 described herein typically have an ordered olivine structure having a plurality of planes defined by zigzag chains and linear chains, where the M atoms occupy the zigzag chains of octahedra and the Li atoms occupy the linear chains of alternate planes of octahedral sites.
In another aspect, the invention provides electrode materials for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte, with or without an electrode separator, where the electrode materials comprise a rhombohedral NASICON material having the formula YxM2(PO4)3, where 0xe2x89xa6xxe2x89xa65. Preferably, the compounds of the invention will be useful as the cathode of a rechargeable electrochemical cell. The alkali ion Y may be inserted from the electrolyte of the battery to the interstitial space of the rhombohedral M2(XO4)3 NASICON host framework as the transition-metal M cation (or combination of cations) is reduced by charge-compensating electrons supplied by the external circuit of the battery during discharge with the reverse process occurring during charge of the battery. While it is contemplated that the materials of the invention may consist of either a single rhombohedral phase or two phases, e.g. orthorhombic and monoclinic, the materials are preferably single-phase rhombohedral NASICON compounds. Generally, M will be at least one first-row transition-metal cation and Y will be Li or Na. In preferred compounds, M will be Fe, V, Mn, or Ti and Y will be Li.
Redox energies of the host M cations can be varied by a suitable choice of the XO4 polyanion, where X is taken from Si, P, As, or S and the structure may contain a combination of such polvanions. Tuning of the redox energies allows optimization of the battery voltage with respect to the electrolyte used in the battery. The invention replaces the oxide ion O2xe2x88x92 of conventional cathode materials by a polyanion (XO4)mxe2x88x92 to take advantage of (1) the larger size of the polyanion, which can enlarge the free volume of the host interstitial space available to the alkali ions, and (2) the covalent Xxe2x80x94O bonding, which stabilizes the redox energies of the M cations with M-O-X bonding so as to create acceptable open-circuit voltages Voc with environmentally benign Fe3+/Fe2+ and/or Ti4+/Ti3+ or V4+/V3+ redox couples.
Preferred formulas for the rhombohedral NASICON electrode compounds of the invention include, but are not limited to those having the formula Li3+xFe2(PO4)3, Li2+xFeTi(PO4)3, LixTiNb(PO4)3, and Li1+xFeNb(PO4)3, where 0 less than x less than 2. It will be understood by one of skill in the art that Na may be substituted for Li in any of the above compounds to provide cathode materials for a Na ion rechargeable battery. For example, one may employ Na3+xFe2(PO4)3, Na2+xFeTi(PO4)3, NaxTiNb(PO4)3 or Na1+xFeNb(PO4)3, where 0 less than x less than 2, in a Na ion rechargeable battery. In this aspect, Na+ is the working ion and the anode and electrolyte comprise a Na compound.
Compounds of the invention having the rhombohedral NASICON structure form a framework of MO6 octahedra sharing all of their corners with XO4 tetrahedra (X=Si, P, As, or S), the XO4 tetrahedra sharing all of their corners with octahedra. Pairs of MO6 octahedra have faces bridged by three XO4 tetrahedra to form xe2x80x9clanternxe2x80x9d units aligned parallel to the hexagonal c-axis (the rhomobhedral [111] direction), each of these XO4 tetrahedra bridging to two different xe2x80x9clanternxe2x80x9d units. The Li+ or Na+ ions occupy the interstitial space within the M2(XO4)3 framework. Generally, YxM2(XO4)3 compounds with the rhombohedral NASICON framework may be prepared by solid-state reaction of stoichiometric proportions of the Y, M, and XO4 groups for the desired valence of the M cation. Where Y is Li, the compounds may be prepared indirectly from the Na analog by ion exchange of Li+ for Na+ ions in a molten LiNO3 bath at 300xc2x0 C. For example. rhombohedral LiTi2(PO4)3 may be prepared from intimate mixtures of Li2CO3 or LiOHxc2x7H2O, TiO2, and NH4H2PO4xc2x7H2O calcined in air at 200xc2x0 C. to eliminate H2O and CO2 followed by heating in air for 24 hours near 850xc2x0 C. and a further heating for 24 hours near 950xc2x0 C. However, preparation of Li3Fe2(PO4)3, by a similar solid-state reaction gives the undesired monoclinic framework. To obtain the rhombohedral form, it is necessary to prepare rhombohedral Na3Fe2(PO4)3 by solid-state reaction of NaCO3, Fe{CH2COOH}2 and NH4H2PO4xc2x7H2O, for example. The rhombohedral form of Li3Fe2(PO4)3 is then obtained at 300xc2x0 C. by ion exchange of Li+ for Na+ in a bath of molten LiNO3. It will be understood by one of skill in the art that the rhombohedral Na compounds will be useful as cathode materials in rechargeable Na ion batteries.
In another aspect of the invention, the rhombohedral NASICON electrode compounds may have the general formula YxM2(PO4)y(XO4)3xe2x88x92y, where 0 less than yxe2x89xa63, M is a transition-metal atom, Y is Li or Na, and X=Si, As, or S and acts as a counter cation in the rhombohedral NASICON framework structure. In this aspect, the compound comprises a phosphate anion as at least part of an electrode material. In preferred embodiments, the compounds are used in the cathode of a rechargeable battery. Preferred compounds having this general formula include, but are not limited to Li1+xFe2(SO4)2(PO4), where 0xe2x89xa6xxe2x89xa61.
The rhombohedral NASICON compounds described above may typically be prepared by preparing an aqueous solution comprising a lithium compound, an iron compound, a phosphate compound and a sulfate compound, evaporating the solution to obtain dry material and heating the dry material to about 500xc2x0 C. Preferably, the aqueous starting solution comprises FeCl3, (NH4)2SO4, and LiH2PO4.
In a further embodiment, the invention provides electrode materials for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte, with or without an electrode separator, where the electrode materials have a rhombohedral NASICON structure with the general formula A3xe2x88x92xV2(PO4)3. In these compounds, A may be Li, Na or a combination thereof and 0xe2x89xa6xxe2x89xa62. In preferred embodiments, the compounds are a single-phase rhombohedral NASICON material. Preferred formulas for the rhombohedral NASICON electrode compounds having the general formula A3xe2x88x92xV2(PO4)3 include, but are not limited to those having the formula Li2xe2x88x92xNaV2(PO4)3, where 0xe2x89xa6xxe2x89xa62.
The rhombohedral NASICON materials of the general formula A3xe2x88x92xV2(PO4)3 may generally be prepared by the process outlined in FIG. 9. Alternatively, Li2NaV2(PO4)3 may be prepared by a direct solid-state reaction from LiCO3, NaCO3, NH4H2PO4xc2x7H2O and V2O3.
In a further aspect, the invention provides a secondary (rechargeable) battery where an electrochemical cell comprises two electrodes and an electrolyte, with or without an electrode separator. The electrodes are generally referred to as the anode and the cathode. The secondary batteries of the invention generally comprise as electrode material, and preferably as cathode material, the compounds described above. More particularly, the batteries of the invention have a cathode comprising the ordered olivine compounds described above or the rhombohedral NASICON compounds described above.